As a result of several well-known tragic accidents and considerable experience with ordnance items exposed to thermal stimulus it has been found necessary to pursue technologies offering more satisfactory solutions to reducing or mitigating reaction response level of munitions subjected to fast cook-off (FCO) and slow cook-off (SCO) threat stimuli. To be in compliance with Insensitive Munitions (IM) requirements, munitions are not permitted to exhibit a reaction response more violent than a burning or deflagration reaction when subjected to a FCO or SCO thermal environment. Likewise, new rocket/missile propulsion systems are required to meet insensitive munitions requirements in prevailing applicable standards. Currently, few propulsion systems available meet the presently applicable FCO requirements and virtually none pass SCO requirements.
The effectiveness of cook-off hazard mitigation systems which cut and/or otherwise vent a rocket motor case prior to ignition of the propellant mass by the endangering thermal stimulus has been demonstrated. In full scale cook-off tests, dramatic reduction in the reaction violence of the motors has been obtained by implementing the active mitigation case venting approach. Nevertheless, cookoff hazard mitigation for rocket motors is a difficult engineering problem, and the technology is still immature, especially in the area of slow cookoff mitigation. No current mitigation systems can effectively and reliably sense and mitigate both fast cookoff and slow cookoff, not to mention intermediate cook-offs. New missile systems currently in the initial stages of design and development are required to meet the insensitive munitions requirements before acceptance for use. Many will not be able to do so without advances in cookoff hazard mitigation technology.
At the present time, the primary problem in the development of a satisfactory case venting cookoff mitigation system is the lack of fully suitable thermal sensor technologies. Mechanical sensors which utilize bimetal, memory metal, and/or wax motor actuators to sense and initiate a mitigation system have been built and tested but are bulky and expensive. Electrically powered thermal sensors, active or passive, generally are not considered capable of meeting design requirements for this application because of one or more of the following reasons: (1) the need for a reliable long-term power source, (2) the need for maintenance, (3) the possibility of system failure or an electrical short resulting in accidental triggering, (4) the difficulty of fool proofing and hardening the system so that it cannot be triggered by aero heating, electromagnetic pulse inputs, or any other stimuli except valid thermal threats, and (5) difficulty assuring electronic component reliability at elevated temperatures in a cookoff environment. For these reasons, all current cookoff hazard mitigation systems are designed around rather primitive thermal sensors which utilize a pyrotechnic charge as the sensing element.
Pyrotechnic thermal sensors can only respond to heating rates at the upper end of the heating rate continuum. Pyrotechnic thermal sensors that are capable of sensing and responding to slow cookoff threats are generally too sensitive to be seriously considered. Thus, current cookoff mitigation systems are unable to sense, respond to, or mitigate hazards presented by intermediate or slow cookoff. As a rule, pyrotechnics also lack precise, reproducible behavior, and their use raises safety issues regarding the mitigation system itself. Some pyrotechnic-based mitigation systems require the implementation of a safe/arming subsystem which adversely impacts complexity, reliability, and cost.